The present invention relates to a control arrangement for a hydraulic brake system in general, and more particularly for a vehicular braking system equipped with an antiskid device.
A hydraulic brake system of this type usually incorporates a master cylinder and at least one wheel brake actuating cylinder, a pressure control valve connected upstream of the wheel brake actuating cylinder to selectively establish connection between the wheel brake actuating cylinder and the master cylinder or a return reservoir, and a valve device comprising a control piston for supplying pressure fluid from a pressure source into the brake circuits. A braking system of this type is disclosed for instance, in commonly owned German application P 30 10 639.9. This system includes a vacuum brake booster having a tandem master cylinder connected thereto and situated downstream thereof. The working chambers of the tandem master cylinder communicate with wheel brake cylinders via pressure lines and via supply valves which are open in their de-energized state. Furthermore, relief valves which are closed in their de-energized state are assigned to the wheel brake actuating cylinders. The relief valves are open during an antiskid control action in response to control commands of an electronic control unit, so that a rapid pressure decrease can take place in the respective wheel brake actuating cylinders. To prevent the fluid enclosed in the working chambers of the master cylinder from being exhausted during the antiskid control operation, it is necessary to replenish the pressure fluid removed during the antiskid control operation from the pressure source. For this purpose, the brake system includes a valve device that makes available at its outlet a pressure corresponding to the pressure in the working chambers of the master cylinder. This device ensures that the pressure fluid removed during the antiskid control action will be replenished from an independent reservoir. In case of need, the pressure fluid is fed into the brake circuits through check valves which are adapted to be opened in opposition to the master cylinder pressure. Connected upstream of the valve device is another valve which is closed in its de-energized state and which is opened as soon as one of the relief valves, which are arranged at the wheel brakes, switches over to its open position. The position of the normally closed other valve, which is assigned to the valve device, is further dependent upon the outlet pressure of the valve device. In this manner, it is ensured that the connection between the pressure source and the valve device will be interrupted only when the pressure in the working chambers of the tandem master cylinder has been completely decreased.
As advantageous as this arrangement may be in many respects, it still leaves much to be desired in other respects. So for instance, it necessitates a comparatively complicated and costly design of the normally closed other valve which is associated with the valve device. Apart from several radial bores, the control piston of the valve device also includes in its interior control channels, which results in a situation where intensive machining of this part is required. As mentioned already, the pressure at the outlet of the valve device exactly corresponds to the pressure built up in the working chambers of the tandem master cylinder.